It will be very useful to detect types and categories of unknown substances with various new technologies. An example is recently using ion mobility technology to detect dangerous articles, such as explosives, or drugs, in the market of safety inspection, to prevent such dangerous articles from entering public places.
Currently, ion mobility detectors (or ion mobility spectrometer) that use ion mobility technology to detect dangerous articles are classified, based on different ion polarities to be detected, into positive-mode ion mobility detectors for detecting positive ions and negative-mode ion mobility detectors for detecting negative ions. The detection coverage (application) of such ion mobility detectors is limited due to positive and negative modes of ions. While most molecules have specific electroaffinity, a few of molecules can produce both positive and negative ions at the same time. Dual-mode ion mobility detectors (or dual-polarity IMS) equipped with respective positive and negative mobility zones have been developed in order to expand the coverage of detection with ion mobility technology. Such ion mobility detectors are large-sized, and have larger detection coverage and higher resolution. The ion mobility detectors in market generally appear as a set of machine, and cost more than single-mode ion mobility detectors.
The conventional dual-mode ion mobility detector primarily consists of an ion source, a positive ion gate, a negative ion gate, two drift tubes (TOF), and two Faraday plates. The simplest configuration is locating the two drift tubes on the respective sides of the ion source. The potential of the ion source is generally ground potential (i.e., potential of zero) since the electric fields of the positive and negative mobility zones have the same direction. The amplitude of a pulsed voltage is decided by quantity of electric charges carried by an ion cluster arriving at the Faraday plates, and usually reflects the number of collected ions. Accordingly, it is possible to determine the particular type of some substance by analyzing variations of the pulsed voltage. To ensure sufficient electric field strength between the Faraday plates and the ion source, the Faraday plates are placed at a high potential of several thousand volts (often around 3,000V), and circuits connected behind the Faraday plates, such as a leadout circuit for the pulsed voltage (often about several millivolts), an amplification circuit and an analog-to-digital conversion circuit for the pulsed voltage, are floating at a high potential of several thousand volts.
Conventionally, transforms are used to transform a high voltage of several thousand volts to the zero potential, that is, setting amplification and shaping circuits at backend as floating at a high voltage of several thousand volts, and then extracting an amplified pulsed electric signal through an isolation device. Since such high voltage up to several thousand volts has a strict requirement on resistance against high voltage, there are only a narrow range of electronic devices that can be selected for the transformer. Moreover, circuits within the transformer and peripheral leadout circuits electrically connected to the transformer are complex. As a result, it is difficult to design and manufacture the leadout circuit for the pulsed voltage on the Faraday plates, leading to difficulties in digitalization and subsequent processing of the pulsed voltage signal.